The present invention generally relates to an atrial defibrillator for applying cardioverting electrical energy to the atria of a human heart in need of cardioversion. The present invention is more particularly directed to a fully automatic implantable atrial defibrillator which exhibits improved safety by reducing the potential risk of induced ventricular fibrillation or tachycardia which may result from the mistimed delivery of cardioverting electrical energy to the atria of the heart. More specifically, the atrial defibrillator of the present invention guards against applying cardioverting electrical energy to the atria of the heart under conditions believed to contribute to induced ventricular fibrillation or tachycardia.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well know in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected ventricular electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistant to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality.
Two such proposed defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these proposed defibrillators require the patient to recognize the symptoms of atrial fibrillation, with one defibrillator requiring a visit to a physician to activate the defibrillator, and the other defibrillator requiring the patient to activate the defibrillator with an external magnet.
An improved atrial defibrillator and lead system which exhibits both automatic operation and improved safety is fully described in U.S. Pat. No. 5,282,837, which issued on Feb. 1, 1994, for ATRIAL DEFIBRILLATOR AND METHOD, and which patent is assigned to the assignee of the present invention and is incorporated herein by reference. As disclosed in the aforementioned referenced patent, synchronizing the delivery of the defibrillating or cardioverting electrical energy to the atria with a ventricular electrical activation (R wave) of the heart is important to prevent induced ventricular fibrillation. Ventricular fibrillation is a fatal arrhythmia which can be caused by electrical energy being delivered to the heart at the wrong time in the cardiac cycle, such as during the T wave of the cycle.
The atrial defibrillator of the aforementioned referenced patent exhibits further improved safety from inducing ventricular fibrillation by generating reliable synchronization signals. This is accomplished by sensing ventricular activations of the heart in a manner which rejects noise as ventricular electrical activations. By providing such noise immunity in R wave detection, reliable synchronization is achieved.
Another measure for reducing the risk of inducing ventricular fibrillation during the delivery of cardioverting electrical energy to the atria of the heart employed by the defibrillator of the aforementioned referenced patent is the reduction of the amount of electrical energy which is passed through the ventricles during cardioversion of the atria. This is achieved by locating the cardioverting electrodes to provide a cardioverting energy path which confines most of the cardioverting electrical energy to the atria of the heart.
Another atrial defibrillator and method which provides a still further improvement to the end of safety and reduction in the risk of inducing ventricular fibrillation during atrial cardioversion or defibrillation is described in U.S. Pat. No. 5,207,219, which issued on May 4, 1993, for ATRIAL DEFIBRILLATOR AND METHOD FOR PROVIDING INTERVAL TIMING PRIOR TO CARDIOVERSION, and which patent is also assigned to the assignee of the present invention and is incorporated herein by reference. As described in that patent, it has been observed that during episodes of atrial fibrillation, the cardiac rate may increase to a high rate. At high cardiac rates, the R wave of each cardiac cycle becomes closely spaced from the T wave of the immediately preceding cardiac cycle. This may lead to a condition known in the art as an "R on T" condition which is believed to contribute to induced ventricular fibrillation if the atria are cardioverted in synchronism with an R wave close to a T wave.
The atrial defibrillator and method described in U.S. Pat. No. 5,207,219 greatly reduces this risk by avoiding applying the cardioverting electrical energy to the atria at those instances when increased vulnerability to ventricular fibrillation may be present. This is accomplished by interval timing prior to applying the cardioverting or defibrillating electrical energy. The time interval between immediately successive R waves is timed and the cardioverting or defibrillating electrical energy is applied when a timed interval is greater than a preselected minimum interval. This provides protection for the increased vulnerability to ventricular fibrillation condition resulting from a high cardiac rate.
It is further observed in U.S. Pat. No. 5,207,219 that during highly variable cardiac rates, a long cardiac cycle can be followed by a relatively short cardiac cycle and that this condition in conjunction with a high cardiac rate may cause a dispersion of refractoriness and also may result in an increased vulnerability to ventricular fibrillation or tachycardia. To provide protection for a highly variable cardiac rate, U.S. Pat. No. 5,207,219 suggests a further condition be applied to the timed interval, requiring the timed interval to also be less than a preselected maximum interval before the cardioverting or defibrillating energy is applied to the atria.
While the minimum interval criteria has proven essential in providing safe cardioversion of the atria, there remains room for further improvement. For example, it has been found that a minimum interval sufficiently long so as to be safe under all conditions, including the length of a previous interval and especially where a highly variable cardiac rate is present, limits the number of cardiac cycles which would satisfy the minimum interval criteria. This correspondingly limits the probability of locating an R wave with which to synchronize the cardioverting energy delivery.
The present invention realizes an improvement by increasing the number of such available cycles for synchronizing the energy delivery, while still maintaining safety. This improvement is accomplished in practicing the present invention by requiring a predefined relationship or criteria to be satisfied between a current cardiac interval (one to be used for synchronization) and the interval immediately preceding the current interval. If that criteria is satisfied, the energy may still be delivered safely even though the current interval is less than a minimum interval considered safe under all conditions.